Electro-optical device

ABSTRACT

An auxiliary capacitor for a pixel of an active matrix type liquid crystal display is provided without decreasing the aperture ratio. A transparent conductive film for a common electrode is formed under a pixel electrode constituted by a transparent conductive film with an insulation film provided therebetween. Further, the transparent conductive film for the common electrode is maintained at fixed potential, formed so as to cover a gate bus line and a source bus line, and configured such that signals on each bus line are not applied to the pixel electrode. The pixel electrode is disposed so that all edges thereof overlap the gate bus line and source bus line. As a result, each of the bus lines serves as a black matrix. Further, the pixel electrode overlaps the transparent conductive film for the common electrode to form a storage capacitor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.09/850,886, filed May 7, 2001 now U.S. Pat. No. 6,985,887, which is acontinuation of U.S. application Ser. No. 09/360,841, filed Jul. 22,1999 now U.S. Pat. No. 6,246,453, which is a continuation of U.S.application Ser. No. 08/881,182, filed Jun. 24, 1997 now U.S. Pat. No.8,982,460.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention disclosed in this specification relates to a configurationof a pixel region of an active matrix type liquid crystal display and,more particularly, to a configuration of an auxiliary capacitorconnected to a pixel electrode in parallel and a configuration of ablack matrix for preventing leakage of light at boundaries betweenadjacent pixels.

2. Description of Related Art

Liquid crystal displays having an active matrix circuit are known. Theyhave a configuration including a plurality of source bus lines fortransmitting image data, a plurality of gate bus lines for transmittingswitching signals in an intersecting relationship therewith, and aplurality of pixels provided at those intersections. Normally,transistors (specifically thin film transistors) are used as switchingelements.

A pixel includes not only a transistor for switching but also a pixelelectrode and has a structure in which the gate electrode, source, anddrain of the transistor are connected to a gate bus line, a source busline, and the pixel electrode, respectively. Distinction between thesource and drain of a transistor is not fixed during the operationthereof and varies depending on signals according to a common definitionof electrical circuits. In the following description, however, the terms“source” and “drain” simply refer to impurity regions provided in atransistor which are connected to a source bus line and a pixelelectrode, respectively.

Each pixel includes one or more transistors. Specifically, two or moretransistors connected in series are advantageous in that leak currentcan be reduced even when the transistors are not selected. Theabove-described definition is also applied to such a case, and nodefinition is given to an impurity region connected to neither a sourcebus line nor a pixel electrode.

A capacitor is formed between a pixel electrode and an electrode whichis opposite to the pixel electrode across liquid crystal. A transistoras described above serves as a switching element for supplying andremoving electrical charge to and from this capacitor.

In an actual operation, however, the capacity of the pixel electrodeportion is too small to store necessary electrical charge for asufficient period of time by itself. It is therefore necessary toprovide a separate auxiliary capacitor.

Such an auxiliary capacitor (also referred to as “storage capacitor”)has been formed between an opaque conductive material such as a metalwhich is separately provided and a pixel electrode or a semiconductorlayer. The gate bus line for the next row has been normally used as theopposite electrode. Although a capacitor formed using a gate bus linehas been sufficient when the area of the pixel is large enough, a gatebus line has been unsuccessful in providing a sufficient capacity byitself when the area of the pixel is small. This has required a gate busline to be expanded to accommodate the area for an electrode of anauxiliary capacitor. Such a structure results in a decrease in theaperture ratio because a pixel includes an area where light is blocked.

It is an object of the invention disclosed in this specification toprovide a configuration wherein an auxiliary capacity is increasedwithout reducing the aperture ratio of a pixel. It is another object ofthe invention to provide a configuration of a black matrix for solvingthe problem of leakage of light that occurs at boundaries betweenadjoining pixels.

SUMMARY OF THE INVENTION

The invention disclosed in this specification is characterized in that:

a pixel includes a pixel electrode comprising a first transparentconductive film;

the pixel electrode overlaps a gate bus line and a source bus line;

a layer of a common electrode comprising a second transparent conductivefilm is provided between the gate bus line and the pixel electrode andbetween the source bus line and the pixel electrode such that it coversthe gate bus line and source bus line; and

the common electrode is maintained at constant potential.

Specifically, the common electrode is disposed such that it covers thebus lines, and the pixel electrode is disposed such that it overlaps thebus lines. Thus, the common electrode and the pixel electrode overlapeach other, and this overlap provides an auxiliary capacitor. Inaddition, while at least one of the electrodes of a conventionalauxiliary capacitor has been formed of an opaque material, both of suchelectrodes are formed of a transparent material according to the presentinvention. Thus, they will not hinder display and the aperture ratio ismaintained.

With the above-described configuration, a black matrix can be formed bysource bus lines and gate bus lines by arranging pixel electrodes in anoverlapping relationship with the source bus lines and gate bus linesand by positioning boundaries between the pixel electrodes on the sourcebus lines and gate bus lines. In general, boundaries between pixelelectrodes of a liquid crystal display are affected by the electricfield of the adjacent electrodes. As a result, at such boundaries, animage can be produced differently from that to be formed by the relevantpixels or leakage of light can occur due to absence of an electricfield.

It is therefore not appropriate to use such boundaries between pixelelectrodes, and a structure is normally employed in which such areas areshaded by a black matrix. It has been conventionally necessary toconfigure a black matrix by a separate layer. For example, the use ofbus lines as a black matrix has been proposed in Japanese unexaminedpatent publication (KOKAI) No. H6-216421 and etc. In practice, however,this results in unstable display because signals on bus lines affectpixel electrodes.

The present invention solves this problem. Specifically, since a commonelectrode is provided between a bus line and a pixel electrode, a signalon the bus line is blocked by the common electrode and hence does notaffect the pixel electrode.

When a top-gate type transistor (a transistor having a structure inwhich the gate electrode is provided on top of a semiconductor layer) isused as a switching transistor, it will be advantageous for stableoperation of the transistor if light is allowed to be incident upon thesubstrate primarily from above, i.e., from the side of the pixelelectrodes because this prevents light from entering the semiconductorlayer under the gate electrode.

The effect of shading can be further improved for more stable operationby providing a film made of the same material as the layer of the sourcebus lines on top of the gate bus lines where the semiconductor layer andthe gate bus lines intersect with each other.

As an insulator to be used between the common electrode and the pixelelectrode, organic resin may be used as well as inorganic materials(e.g., silicon oxide and silicon nitride).

Especially, a flat insulation layer formed by means of spin coating orthe like will be effective in reducing surface irregularity for improveduniformity of an electric field applied to liquid crystal molecules.

Materials usable for the transparent conductive film according to theinvention disclosed in this specification include ITO (indium tinoxide), SnO₂, and materials mainly composed of those materials as well.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1E are sectional views showing fabrication stepsaccording to a first embodiment of the present invention.

FIGS. 2A through 2D illustrate the configuration of wiring and the likeaccording to the first embodiment of the invention.

FIGS. 3A through 3E are sectional views showing fabrication stepsaccording to a second embodiment of the present invention.

FIGS. 4A through 4D illustrate the configuration of wiring and the likeaccording to the second embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A first embodiment of the present invention will now be described. FIGS.1A through 1E and FIGS. 2A through 2D show a configuration of a pixel ofan active matrix type liquid crystal display which employs the inventiondisclosed in this specification. FIGS. 1A through 1E are schematicsectional views showing fabrication steps according to the presentembodiment, and FIGS. 2A through 2D show the configuration of each of abus line, a common electrode, pixel electrodes, a semiconductor layer,and the like according to the present embodiment. The reference numbersin FIGS. 2A through 2D are in correspondence with those in FIGS. 1Athrough 1E. FIGS. 1A through 1E are conceptual views and are not exactlyidentical to FIGS. 2A through 2D in configuration.

Further, FIGS. 1A through 1E and FIGS. 2A though 2D show a configurationof only a substrate on which a thin film transistor is provided. Inpractice, there is provided another substrate opposite thereto (oppositesubstrate), and liquid crystal is held between the opposite substrateand the substrate shown in FIGS. 1A through 1E with a gap of several μmtherebetween.

The fabrication steps will now be described with reference to FIGS. 1Athrough 1E. As shown in FIG. 1A, a semiconductor layer (active layer) 12of a transistor is provided on a glass substrate 11 having an underlyingsilicon oxide film (not shown).

The active layer 12 is formed by a crystalline silicon film which hasbeen crystallized by heating an amorphous silicon film or by irradiatingthe same with laser beams. A gate insulation film 13 is formed so as tocover the active layer 12. The gate insulation film 13 is preferablymade of silicon oxide or silicon nitride and, for example, a siliconoxide film formed using a plasma CVD process may be used. A gate busline (gate electrode) 14 made of an aluminum-titanium alloy is formed onthe gate insulation film using a well known sputtering process (FIG.1A).

The configuration of this circuit in this state is shown in FIG. 2A.

Next, a well known ion doping process is performed using the gate busline as a mask to introduce N- or P-type impurities in the active layer,thereby forming a source 15 and a drain 16.

After the impurities are introduced, thermal annealing, laser annealingor the like may be performed to activate the impurities (torecrystallize the semiconductor film) if required.

After the above-described steps, a silicon nitride film (or a siliconoxide film) 17 is deposited by means of a plasma CVD process. It servesas a first layer insulator (FIG. 1B).

Next, contact holes are formed in the first layer insulator 17 such thatthey reach the source 15 and drain 16. Then, a well known sputteringprocess is performed to form a multi-layer film of titanium and aluminumwhich is in turn etched to form a source bus line 18 and a drainelectrode 19.

After the above-described steps, a silicon nitride film (or a siliconoxide film) 20 is deposited by means of a plasma CVD process. It servesas a second layer insulator (FIG. 1C).

The configuration of the circuit in this state is shown in FIG. 2B.

Next, a spin coating process is performed to form a first organic resinlayer 21. The organic resin layer is formed to have a flat uppersurface. Then, a well known sputtering process is performed to form anITO film which is in turn etched to form a common electrode 22 (FIG.1D).

The configuration of the circuit in this state is shown in FIG. 2C. Thecommon electrode is shaded in FIG. 2C to show its position clearly. Asapparent from FIG. 2C, the common electrode is formed so as to cover thesource bus line and gate bus line.

Further, a spin coating process is performed to form a second organicresin layer 23. Then, a well known sputtering process is performed toform an ITO film which is in turn etched to form pixel electrodes 24 aand 24 b. The pixel electrode 24 b is a pixel electrode for thetransistor as described above, and the pixel electrode 24 a is a pixelelectrode adjacent thereto. Capacitors 25 a and 25 b are respectivelyformed at regions where the pixel electrodes 24 a and 24 b overlap thecommon electrode 22 (FIG. 1E).

The configuration of the circuit in this state is shown in FIG. 2D. InFIG. 2D, the pixel electrodes and the regions where the pixel electrodesoverlap the common electrode (regions where the capacitors are located)are shaded to show their positions clearly. As apparent from FIG. 2D,the pixel electrodes are formed so as to overlap the source bus line andgate bus line. As a result, the boundaries of the pixel electrodes areall located on the bus lines which consequently serve as a black matrix(FIG. 2D).

A second embodiment of the present invention will now be described.FIGS. 3A through 3E and FIGS. 4A through 4D show a configuration of apixel of an active matrix type liquid crystal display which employs theinvention disclosed in this specification. FIGS. 3A through 3E areschematic sectional views showing fabrication steps according to thepresent embodiment, and FIGS. 4A through 4D show the configuration ofeach of a bus line, a common electrode, pixel electrodes, asemiconductor layer, and the like according to the present embodiment.The reference numbers in FIGS. 4A through 4D are in correspondence withthose in FIGS. 3A through 3E. FIGS. 3A through 3E are conceptual viewsand are not exactly identical to FIGS. 4A through 4D in configuration.

As shown in FIG. 3A, a semiconductor layer (active layer) 32 of atransistor is provided on a glass substrate 31 having an underlyingsilicon oxide film (not shown). A gate insulation film 33 is formed soas to cover the active layer 32. A gate bus line (gate electrode) 34made of an aluminum-titanium alloy is formed on the gate insulation film(FIG. 3A).

The configuration of this circuit in this state is shown in FIG. 4A.Unlike the first embodiment, the gate bus line of the present embodimentis configured to be reduced in width at the region of the gate electrodeof the transistor (FIG. 4A).

Next, N- or P-type impurities are introduced to form a source 35 and adrain 36. After the above-described steps, a first layer insulator 37which is a silicon nitride film (or a silicon oxide film) is deposited(FIG. 3B).

Next, contact holes are formed in the first layer insulator 37 such thatthey reach the source 35 and drain 36. Then, a source bus line 38, adrain electrode 39, and a protective film 40 are formed. After theabove-described steps, a second layer insulator 41 which is a siliconnitride film (or a silicon oxide film) is deposited (FIG. 3C).

The configuration of the circuit in this state is shown in FIG. 4B. Theprotective film 40 is insulated from the source bus line 38, the drainelectrode 39, and other wiring and electrodes to be at floatingpotential. Such a protective film 40 is effective in blocking lightincident upon the transistor from above (FIG. 4B).

Next, a common electrode 42 is formed by an ITO film. Further, anorganic resin layer 43 is formed (FIG. 3D).

The configuration of the circuit in this state is shown in FIG. 4C. Thecommon electrode is shaded in FIG. 4C to show its position clearly. Asapparent from FIG. 4C, the common electrode is formed so as to cover thesource bus line and gate bus line. Strictly speaking, it is notessential to cover the protective film 40 with the common electrode.This is because there is a bare possibility that the protective film hassome influence on the pixel electrodes as it is at floating potential.In the present embodiment, however, the protective film 40 is alsocovered by the common electrode 42 as illustrated (FIG. 4C).

Then, pixel electrodes 44 a and 44 b are formed by ITO films. The pixelelectrode 44 b is a pixel electrode for the transistor as describedabove, and the pixel electrode 44 a is a pixel electrode adjacentthereto. Capacitors 45 a and 45 b are respectively formed at regionswhere the pixel electrodes 44 a and 44 b overlap the common electrode 42(FIG. 3E).

The configuration of the circuit in this state is shown in FIG. 4D. InFIG. 4D, the pixel electrodes and the regions where the pixel electrodesoverlap the common electrode (regions where the capacitors are located)are shaded to show their positions clearly. As apparent from FIG. 4D,the pixel electrodes are formed so as to overlap the source bus line andgate bus line. As a result, the boundaries of the pixel electrodes areall located on the bus lines which consequently serve as a black matrix(FIG. 4D).

By forming an electrode opposite to a pixel electrode that constitutesan auxiliary capacitor using a transparent conductive film, a greatauxiliary capacitor can be formed without decreasing the aperture ratio.In addition, a source bus line and a gate bus line can be used as ablack matrix. More particularly, the present invention is effectiveespecially in improving an aperture ratio when the pixel is small and,especially, with design rules kept unchanged. As described above, thepresent invention has advantages from an industrial point of view.

It should be understood that the foregoing description is onlyillustrative of the invention. Various alternatives and modificationscan be devised by those skilled in the art without departing from theinvention. Accordingly, the present invention is intended to embrace allsuch alternatives, modifications and variances which fall within thescope of the appended claims.

1. An active matrix display device comprising: a thin film transistorformed over a substrate; a first inorganic insulating film formed overthe thin film transistor; a source line formed over the first inorganicinsulating film; a second inorganic insulating film formed over thesource line; a first organic insulating film formed over the secondinorganic insulating film; a common electrode formed over the firstorganic insulating film; a second organic insulating film formed overthe common electrode; and a pixel electrode formed over the secondorganic insulating film.
 2. An active matrix display device according toclaim 1, wherein each of the first and second inorganic insulating filmscomprises at least one material selected from the group consisting ofsilicon oxide and silicon nitride.
 3. An active matrix display deviceaccording to claim 1, wherein the pixel electrode comprises at least onematerial selected from the group consisting of ITO and SnO₂.
 4. Anactive matrix display device comprising: a thin film transistor formedover a substrate; a first inorganic insulating film fanned over the thinfilm transistor a source line formed over the first inorganic insulatingfilm; a second inorganic insulating Film formed over the source line; afirst organic insulating film formed over the second inorganicinsulating film; a common electrode formed over the first organicinsulating film; a second organic insulating film formed over the commonelectrode; and a pixel electrode formed over the second organicinsulating film, wherein the pixel electrode overlaps the source linewith the common electrode interposed therebetween.
 5. An active matrixdisplay device according to claim 4, wherein each of the first andsecond inorganic insulating films comprises at least one materialselected from the group consisting of silicon oxide and silicon nitride.6. An active matrix display device according to claim 4, wherein thepixel electrode comprises at least one material selected from the groupconsisting of ITO and SnO₂.
 7. An active matrix display devicecomprising; a thin film transistor formed over a substrate; a firstinorganic insulating film formed over the thin film transistor, a sourceline formed over the first inorganic insulating film; a second inorganicinsulating film formed over the source line; a first organic insulatingfilm formed over the second inorganic insulating film; a commonelectrode formed over the first organic insulating film; a secondorganic insulating film formed over the common electrode; and a pixelelectrode formed over the second organic insulating film, wherein thesecond organic insulating film has a leveling surface.
 8. An activematrix display device according to claim 7, wherein each of the firstand second inorganic insulating films comprises at least one materialselected from the group consisting of silicon oxide and silicon nitride.9. An active matrix display device according to claim 7, wherein thepixel electrode comprises at least one material selected from the groupconsisting of ITO and SnO₂.
 10. An active matrix display devicecomprising: a thin film transistor formed over a substrate; a firstinorganic insulating film formed over the thin film transistor; a sourceline formed over the first inorganic insulating film; a second inorganicinsulating film formed over the source line; a first organic insulatingfilm formed over the second inorganic insulating film; a commonelectrode formed over the first organic insulating film; a secondorganic insulating film formed over the common electrode; and a pixelelectrode formed over the second organic insulating film, wherein thepixel electrode overlaps the common electrode with the second organicinsulating film interposed therebetween.
 11. An active matrix displaydevice according to claim 10, wherein each of the first and secondinorganic insulating films comprises at least one material selected fromthe group consisting of silicon oxide and silicon nitride.
 12. An activematrix display device according to claim 11, wherein the pixel electrodecomprises at least one material selected from the group consisting ofITO and SnO₂.
 13. An active matrix display device comprising: a thinfilm transistor formed over a substrate; a first inorganic insulatingfilm formed over the thin film transistor; a source line formed over thefirst inorganic insulating film; a second inorganic insulating filmformed over the source line; a first organic insulating film formed overthe second inorganic insulating film; a common electrode formed over thefirst organic insulating film; a second organic insulating film formedover the common electrode: and a pixel electrode formed over the secondorganic insulating film; wherein the common electrode overlaps thesource line with the first organic insulating film and the secondinorganic insulating film interposed therebetween.
 14. An active matrixdisplay device according to claim 13, wherein each of the first andsecond inorganic insulating films comprises at least one materialselected from the group consisting of silicon oxide and silicon nitride.15. An active matrix display device according to claim 13, wherein thepixel electrode comprises at least one material selected from the groupconsisting of ITO and SnO₂.